1. Field of the Invention
The invention relates to a method and a device for milking an animal, by the use a milking machine, which comprises at least one pair of teatcups which each comprising a liner for receiving a teat of the animal, wherein these liners internally are subjected to a subpressure for extracting milk from the respective teats, and which comprises a pulsator arranged, via a pulsation chamber of each teatcup, to subject the liners of the teatcup to a cyclically pulsating pressure from the outside, which pressure has a cycle with a time period T, wherein said cycle comprises
a first opening phase a, which has a first time period t3 and during which the pressure in the pulsation chamber drops from a first pressure level to a second pressure level,
a second open phase b, which has a second time period tb and during which the pressure in the pulsation chamber is essentially constant at the second pressure level,
a third closing phase c, which has a third time period tc and during which the pressure in the pulsation chamber rises from the second pressure level to the first pressure level,
a fourth closed phase d, which has a fourth time period td and during which the pressure in the pulsation chamber is essentially constant at the first pressure level.
2. Description of the Prior Art
Such methods and devices are previously well known and are used by conventional milking machines. A conventional milking procedure may comprise an initial milking period I, during which the milk flow starts, which period is arranged to stimulate and massage the teat for the purpose of starting the Milk flow. Thereafter, a main milking period II follows and finally a terminating milking period Ill. The initial milking period may according to known technique be performed with a faster pulsation than the main milking period in order to increase the stimulating effect, The most common type of pulsator used has a ratio, i.e. a proportion between an open and a closed position from 70:30 to 60:40. Usually, such a pulsator has two outlets, which give a cyclically pulsating pressure to a pair of teatcups. The cyclically pulsating pressure is phase displaced 180xc2x0 between these two teatcups, so that an open phase of one of the teatcups essentially occur at the same time as a closed phase of the other teatcup. In such a way it is possible to decrease the fluctuation of the vacuum level of the milking member and use the capacity of the existing milking equipment at maximum. However, since at a stimulating initial milking period has a faster pulsation, the time periods of the open and closed phases decrease while the opening and closing phases have an essentially constant time period, which first of all is determined by the properties of the liner and the air volumes at the pulsation side. Therefore, the opening and closing phases take up a relatively greater part of the time period of the total cycle. It is optimal with a ratio of 50:50 for an open/closed proportion during such an increased pulsation speed. It depends on the fact that, the closed phase will, during the faster pulsation at for example a ratio of 70:30, be very short and possibly not even occur.
A problem, which arises at a ratio of 50:50 and with a phase displacement of 180xc2x0, is that an opening phase and a closing phase occur at the same time for a pair of teatcups. This results in a pressure difference and thus an air flow arising between the internal spaces of the two liners, wherein milk in form of droplets may be sucked from one teat via the milk claw and up through the short milk tube leading to the other teatcup and into its liner and there hit the other teats This flow of milk from one teatcup to another is called xe2x80x9ccross-flowxe2x80x9d and may occur at a very high speed. The droplets may contain bacteria, which may penetrate into the teat canal of the teat and thereby cause infections.
The object of the present invention is to provide a method and a device which make it possible to avoid the occurrence of a flow of milk between two teatcups, so-called cross-flow, especially in connection with a fast pulsation of a pulsator and with a ratio of about 50:50.
This object is achieved according to the invention by controlling, when both the second Tb and fourth Td time periods are less than the half time period T for the total cycle, the cyclically pulsating pressures of the respective pulsation chambers in such a way that a flow between the two teatcups of the respective pair is prevented. This control may be performed in several ways, i.e. by a variation of the phase displacement between the two cycles or by a suitable variation of the pulsation speed for the two cycles.
According to a preferred embodiment of the invention, the cyclically pulsating pressures of the respective pulsation chambers are controlled in such a way that the first opening phase of one of the teatcups does not occur at the same time as the third closing phase of the other teatcup. Above all, when these phases coincides, the risk for said flow or milk between the two teatcups occurs. The most practical is that the including cycles have an essentially equal time period with respect to the included phases. Thereby, the cyclically pulsating pressures of the respective pulsation chamber may be controlled so that they have a phase displacement which prevents an opening and closing phase from occurring at the same time, wherein the phase displacement is so controlled that it does not exceed a maximum phase displacement Fmax, wherein
Fmax=(td/T)*360xc2x0 if td less than tb, and
Fmax=(tb/T)*360xc2x0 if tb less than td.
These conditions may in an easy manner be derived mathematically, with the condition that said cycles have identical time periods, which only are phase displaced. Advantageously, the cyclically pulsating pressures of the respective pulsation chambers are controlled to be so close to said maximum phase displacement as possible, wherein said phase displacement is controlled to be at least greater than Fmaxxe2x88x9220xc2x0.
According to another advantageous embodiment of the invention, the pulsation frequency is controlled to be equal to or less than 200 cycles per minute. Thereby, the cyclically pulsating pressures of the respective pulsation chambers may be controlled to have a constant phase displacement in the interval 80xc2x0-100xc2x0. In this case, a phase displacement of 90xc2x0 may be suitable. Thereby, no risk of said flow between the two teatcups arising occurs as long as the pulsation frequency is less than 200 cycles per minute. If the pulsation frequency is controlled to be between 200-400 cycles per minute, a constant phase displacement ought to be controlled to be in the interval 35xc2x0-55xc2x0, preferably the phase displacement 45xc2x0 may be selected
According to a preferred embodiment of the invention, the method is arranged to be performed when the first pressure level is an atmospheric pressure and when the second pressure level is a subpressure of about 50 kPa, i.e. normal pressure levels for milking machines.
According to another advantageous embodiment of the invention, the method is arranged to be performed during the initial massage period I which occurs before the main milking period II.
The invention is also related to a device for performing the method above. The device comprises control means, which are arranged to control the cyclically pulsating pressures according to the above-mentioned method. Thereby, the control means may comprise a control unit which by knowledge of the pulsation speed of the included cycles, i.e. time period and other features, influences the pulsator so that an opening phase does not occur at the same time as a closing phase of a pair of the teatcups. The control unit may be a calculating unit, which controls the cyclically pulsating pressure of the respective pulsation chambers so that they have a phase displacement which does not exceed a maximally calculated phase displacement Fmax. The control unit may also be designed in a more simple manner so that it controls the cyclically pulsating pressures of the respective pulsation chambers to have a constant phase displacement within a pulsation frequency range. This constant phase displacement may be 90xc2x0 at a pulsation frequency under 200 cycles per minute and 45xc2x0 at a pulsation frequency between 200-400 cycles per minute.
Furthermore, said control means may be provided to perform said control during an initial massage period I which occurs before the main milking period II.